/Engine/Source/Runtime/Engine/Classes/PhysicsEngine/ClusterUnionReplicatedProxyComponent.h
1) It lets us avoid adding any additional overhead into theUPrimitiveComponent. 2) It lets the replicated information have the same net relevancy as the actor being added to the cluster union rather than having the same net relevancy as the cluster union (i.e. in the case of replicating this data in an array in the ClusterUnionComponent). 3) It lets us pinpoint what exactly is being added/removed (vs if all this data was stored in an array) which lets us be a bit more efficient in terms of modifying the cluster union.
class UClusterUnionReplicatedProxyComponent : public [UActorComponent](API\Runtime\Engine\Components\UActorComponent)